Electrical properties of thermally stable Pt/Re/Au ohmic contacts to p-type GaN

Thermally stable ohmic contact to moderately doped p-GaN:Mg (1.13×10¹⁷ cm^-3) have been investigated using Pt (20 nm)/Re (30 nm)/Au (80 nm) metallization scheme. It is shown that annealing the samples at 600 °C for 1 min in a N₂ ambient improves the I-V characteristics of the as-deposited contact. The 600 °C contact produces a specific contact resistance of 1.4×10^-3 Ωcm². However, annealing at 800 °C results in the degradation of the I-V characteristics. It is further shown that the surface morphology of the contact annealed at 600 °C is very smooth with a root-mean-square roughness of 1.4 nm and that the contact resistivity of the sample annealed at 600 °C for 30 min is comparable to that of the 1 min annealed sample. Glancing angle X-ray diffraction and Auger electron microscopy are used to characterise the interfacial reactions between the Pt/Re/Au layers and the p-GaN and hence to understand the annealing dependence of ohmic behaviour.